This invention relates to processes for the production of methanol and other oxygenated hydrocarbons from feedstocks containing carbon. More specifically, the invention relates to the production of these compounds from a synthesis gas produced by the gasification with oxygen of such carbon containing feedstocks.
Methanol may be industrially synthesized from a synthesis gas containing hydrogen, carbon monoxide, and carbon dioxide, along with small amounts of inert gases such as methane, argon, and nitrogen. The conversion follows one of two reactions: EQU 2H.sub.2 +CO.fwdarw.CH.sub.3 OH EQU 3H.sub.2 +CO.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O
Methanol synthesis gas may be produced by any one of several known industrial processes, such as catalytic steam reforming, as exemplified by the process described in U.S. Pat. No. 2,829,113; catalytic oxygen reforming, such as, for example, the process described in French Pat. No. 1,158,617; and partial oxidation, such as, for example, the process described in U.S. Pat. No. 3,962,300.
When the feedstock is a light hydrocarbon such as natural gas, it is usually subjected to steam reforming to produce the synthesis gas required for methanol, and the reformed gas has a large excess of hydrogen above the stoichiometric requirement for the synthesis gas. When the feedstock is a heavy hydrocarbon such as a fuel oil, the most common method in use for producing the synthesis gas is the partial oxidation process, such as described in U.S. Pat. No. 3,962,300. Such a partial oxidation process produces a raw synthesis gas containing CO and CO.sub.2 in excess of stoichiometric requirements. Accordingly, some amount of carbon oxides (usually only CO.sub.2) must be removed from the system to correct for this excess. In the conventional processes, the most common correction method is to subject at least part of the gas produced by gasification to a shift conversion reaction, and then remove CO.sub.2 and H.sub.2 S from the gas to obtain a purified synthesis gas having the stoichiometric composition, and a very low methane content of less than about 0.6 mole percent.
When the feedstock is coal, the processing schemes that have been proposed or used are similar to that for fuel oil described above. The coal is gasified with oxygen in an adiabatic reactor, at a temperature above 1,400.degree. C., giving a raw synthesis gas with a methane content of less than about 0.6 mole percent.
A disadvantage of conventional schemes for producing synthesis gas for the production of methanol and other oxygenated organic compounds from fuel oil or coal feedstocks is that a very large amount of oxygen is necessary to obtain the high temperature (in the range of 1,300.degree. C. to 1,600.degree. C.) required, and to achieve the required methane content of less than about 0.6 mole percent on a dry basis in the effluent gas from the gasifier. A second disadvantage is that the purge gas from the synthesis reactor, which must be removed from the gases recycled to the synthesis reaction to prevent hydrocarbons and other inerts from reaching unacceptable levels in the reactor, includes some unreacted H.sub.2 and CO. The removal of these potential reactants reduces the overall efficiency of the process. In addition, the shift conversion reaction that is performed on the raw synthesis gas is an expensive operation due to the high CO concentration in the gas produced by gasification of the feedstock.